The exhaust casing of a turbomachine is mounted downstream from a turbine and generally comprises two coaxial cylindrical walls, respectively a radially inner wall and a radially outer wall, which walls are interconnected by radial arms, the inner wall being surrounded by a cylindrical jacket for guiding a flow of exhaust gas coming from the turbine.
The cylindrical jacket has its downstream end fastened to the inner wall of the exhaust casing, and at its upstream end it has a radial annular portion that extends freely towards the axis of the turbomachine so that the cylindrical jacket and the inner wall of the exhaust casing together define a cavity, commonly referred to as a hub cavity.
This cavity is open at the inner end of the radial annular portion of the cylindrical jacket.
As a result, air coming from upstream flows in the hub cavity, this air penetrating into the cavity via its upstream opening and being taken from between the high-pressure and low-pressure compressors of the turbomachine, thereby having a negative influence on the fuel consumption of the turbomachine.
This flow of cool air in the hub cavity tends to cool the inner wall of the exhaust casing and the radially inner ends of the radial arms of said casing, while the radially outer portions of the arms are maintained at relatively high temperatures by the flow of exhaust gas. This leads to a large thermal gradient in the radial arms that can harm their lifetime.
In addition, because its radial annular portion is free, the cylindrical jacket presents modes of vibration that correspond substantially to the frequencies of the rotor(s) of the turbomachine, and can thus enter into resonance with the rotor(s), thereby generating strong vibration that can harm the lifetime of the cylindrical jacket.